Pitch changing mechanism



Jan. 8, 1946.

f- W. CALDWELL ETAL PITCH CHANGING MECHANISM FilecLJan. 15, 1941 4 Sheets-Sheet '1 INVENTORS flea/W141; Oman 51.1..

EELE Mae-171v. EDWIN E.

4 Sheets-Sheet 2 F. w. CALDWELL ET AL PITCH CHANGING MECHANISM Filed Jan. '15, 1941 m. L .5. w o mm M W H a Jan. 8, 1946'.

Jan. 8,-1946. F. w. CALDWELL EFAL 2,392,364

H CHANGING MECHANISM Filed Jan. 15, 1941 4 Sheets-Sheet 3 NNNNNNN R5 Patented Jam-8, 1946 PITCH CHANGING MECHANISM Frank W. Caldwell and Eric Martin, West Hartford, Conn., and Edwin E. Vincent, Springfield,

Mass" assignors to United Corporation,

East Hartford, onn., a corporation of Delaware Application January 15, 1941, Serial No. 314,440

4 Claims.

Thi invention relates to improvements in aeronautical propellers and has particular reference to an improved'pitch changing mechanism for a controllable-pitch aeronautical propeller.

- Anobject of the invention resides in the provision of an improved pitch changing mechanism of' the character indicated of reduced weight and improved accuracy for constant-speed control. A further object resides in the provision of an improved pitch changing mechanism of the character indicated which is completely and instantly reversible to changethe p p ller pitch in either direction regardless of centrifugal twisting moment; and other forces acting on the propeller blades.

A still further object resides in the provision of an improved pitch changing mechanism of the character indicated which is highly stable in operation and is eilective to maintain the propeller blades locked in adjusted position except when the mechanism is actuated to change the pitch position of the blades.

Other objects and advantages will be more particularly pointed out hereinafter or will become reference numerals are utilized to designate similar parts throughout, there is illustrated a suitable mechanical arrangement for the purpose of disclosing the invention. It is to be imderstood, however, that the drawings are for the purpose of illustration only and are not to be taken as limiting or restricting the invention since it will be apparent to those skilled in the art that various changes in the illustrated arrangement may be resorted to without in any way exceeding the scope of the invention.

In the drawings.

Fig. 1 is a longitudinal sectional view through a pitch changing mechanism constructed according to the invention.

' Fig. 2 is a longitudinal sectional view through glie iiitch changing motor shown in elevation in Fig. 3 is an end elevational-view of the motor shown in Fig. 2 looking at the left hand end-of the motor as shown in Fig. 2.

Fig. 4 is a transverse sectional view on the line 4-4 of Fig. 2 looking in the direction of the arrows.

Fig. 5 is a transverse sectional view on the line 5-5 of Fig. 2 looking in the direction of the arrows.

Fig. 6 is a transverse sectional view on'the line H of Fig. -2 looking in the direction of the arrows.

Fig. 7 is a transverse sectional view similar to Fig. 4 showing the motor control valves ina different operativeposition from that illustrated in Fig.4 and in the same operative position as is shown in Fig. 8.

Fig. 8 is a transverse sectional view similar to Fig. 5 showing the motor control valve in a dif ferent operative position from that illustrated in Fig. 5.

Fig. 9 is a longitudinal sectional view,.similar to Fig. 2, of the left hand portion of the pitch changing motor taken on the line 8-0 of Hg. 3

and showing the motor control valve in the operative position shown in Fig. 5.

Fig. 10 is a longitudinal sectional view of the- Fig. 11 is a front-elevational view of a portionof the reduction gear mechanism incorporated between the pitch changing motor shown in Fig.

2 and the propeller blades.

Fig. 1-2 is a transverse sectional view on the line iI-II of Fig. 1' showing a blade pitch locking mechanism. and

Fig. 13 is a diagrammatic view of the propeller support and the oil line connections for leading hydraulic fluid to and away from the propeller. Referring to the drawings in detail and particularly to Fig. 13, the propellerincludes a hub member generally indicated at ill in which are rotatably mounted a plurality of blades, one of which is generally indicated at 12, and a hub carried pitch changing mechanism, generally indicated at it. The propeller is carried upon and driven by a propeller shaft l6 projecting nsm;

an engine "through a suitable bearing 20, the hub member it being secured on the shaft it by suitable means, such as locating cones one of which is indicated at 22, and a splincd driving connection 2|. Hydraulic fluid under and the space between the tube and the drive shaft may be connected to the valve it through a similar conduit 81. 38 may be connected to the opposite side of the Anenginedrivenpmnp.

valve 34in such a manner that the valve will connect the outlet of the pump with one or the conduits 32 or 31 and the intake of the pump with the other conduit, the relation of the conduits to the pump outlet and intake being determined by the position of the plunger 46 of the valve 34 as controlled by the speed responsive governor 36.

- hub, the pump 38 and the lines connecting these.

devices. If found desirable, a make up reservoir 39 may be connected with the pump throu h a suitable check' valve in-whichcase the operating fluid supply for the pitch changing mechanism would still be separate and independent of the engine lubricating oil supply, at least for governor controlled operation of the pitch changing mechanism.

A'separate motor driven pump 4| may be provided for feathering and unfeatherlng the propeller. This pump may hav its intake connected through a conduit 42 .with the make up reservoir 36 or with the engine oil sump 43and its outlet connected through a conduit 44 and pressure operated valve 45 with one of the conduits 32 or 31 leading to. the pitch chan ing mechanism. In the illustration, thevalve 45 is shown as having a plunger 46 urged by a compression spring 41 I of the hub I6 by'suitable means, such as the andesired, anti-friction elements 12 may be-interthe remainder of the motor by suitable means,

such as the screw threads 66 and locking pins 62. The sleeve 63 is provided at its forward end, that is, the right-hand nd as viewed in Fig. '1, with an integral radial extension 64 the outer portion of which is secured to an annular support 66 in the form of a relatively heavy annular rin or cylinder secured at one end to the forward end nular nut 66 which is screw threaded into a flange 16 formed on the hub and is provided with an annular internal shoulder cooperating with an opposed external shoulder on the member 66. If

posed between these shoulders to facilitate turn:- ing of the nut 66 relative to the member 66 and the nut may be locked in place by suitable means,

- such as the locking pins 14. The flange 64 is seto a position in which it connects the conduits 32 and 31 withthe valve 34 and blocks of! the conduits 44. when the pump H is operated the .plunger 46 will be moved by the fluid pressure,

exerted by this pump against the force of the spring 41 to a position in which it blocks off both conduits 32 and 31 from the valve 34 and connects the conduit 44 with the conduit 3'! through the bypass channel 48. At the same time the channel conduit 32 will be connected with the drain conduit 2I3 to connect one side of the pitch I changing motor with drain so that the motor may continue to operate in a pitch increasing direc ings 88 and 89.

tion until maximum high pitch of the propeller is reached.

' At its end adjacent the pitch changing mechanism the tube 26 may project into a plug member 46 secured in the end of the drive shaft-I6 by a nut 56 screw threaded into the end of the shaft and'both the plug and the nut maybe provided with apertures 5! connected with one side of thepitch changing motor and leading into the space between the conduit -26 and the in- .terior of the shaft l6.

- the detents 54 and bearing against the opposite end of the casing 56 to secure the motor against rotation and endwise movements relative to the cage. The casing 56 closely surrounds the working parts of the motor and is rigidly secured to cured to the'member 66 by means of a clamping nut I6screw threaded into the open end of the member 66 and bearing against the outer edge of a reduction gear spider "which in turn bears against the member 64 and force this member against the adjacent face of an annular gear member 86-which is provided with an external shoulder cooperating with an internal shoulder provided in the member 66 to limit assembly movement of the member 66 through the member 66, and with cooperating splines, as indicated at 82, which maintain the member 86 against rotation relative to the member 66. With this arrangement the motor supporting sleeve 56 is rigidly mounted in the support66 and held against rotation relative to the support by suitable means such as dowel pins, not illustrated, between the member 66 and the flange 64; However, this sleeve may be further supported by extending it with a bearin fit into the main pitch changing gear 84' which gear is rigidly centered relative'to the-member 66 by means of the annular gear member 86 and the annular anti-friction hear- The motor is provided with a drive shaft 66 which projects into an anti-friction. bearing 62 provided in a gear member 64' mounted on the shaft 96 and having a bearing support 96 in an axially offset portion 66 of the bearing spider I8 spaced outwardly from the remainder of the spider 18 along the axis of the shaft 66. Zll'he shaft 96 is tubular; as is particularly shown in Fig. 2, and at its outer end, opposite the motor 52, it is provided with diametrically opposite extensions, as indicated at I66 and I62. These extensions project through apertures, slightly larger than the extensions, provided in a transverse wall I64 of the gear member 64 and into a relatively fixed race member I66.

A cam I66 integral with the wall I64 lies within the race member I66 between the extensions I66 and I62. Movable elements, such as the balls 6, Big. 12, are disposed between the cam I66 and the race I66 in the spaces between the projections I66 and I62 and are forced into wedging relation between the cam and the race member by the compression springs H2, with the object that the gear is always locked against rotation relative to the relatively fixed member I66 except gears, two or which are indicated at I u and m. The axle of each spur gear is mounted at its ends in suitable anti-friction bearings carried in the of a cage, generally indicated at I24, rotatably supported at each end upon the sleeve 53 by the annular anti-friction bearings I24 and I28.

The cage I24 carries one or more axles, as in-.

dicated at I and I32, and upon each axle there I is mounted a double planet gear, as indicated at I34 and I34, respectively, in Fig. 1. One portion the teeth of the internally toothed ring gear 80 which is fixed in the supporting ring 88 and the other portion of each planet gear meshes with the teeth or the internally toothed ring gear at which is rotatably supported in the support ring 88 by the annular anti-friction bearings 88 and 89 and ilxed to the gear 44 against rotation relative thereto. The two ring gears 00 and 88 are of substantially the same diameter but one of them has a slightly less number of teeth than the other. For example, if a very high gear reduction is de-- sired, one of the ring gears may have one tooth less than the other ring gear while both parts of each planet gear have exactly the same number the movable ring gear will have moved a distance of one tooth in one direction or the other and the reduction ratio will be one ring gear tooth for each revolution of the cage I24. With the additional reduction provided between the gear 94 and the gears II. and H4 and the relatively high reduction between the gears H8 and I24 and the ring gear I22, it is apparent that an extremely high ratio can be obtained between the motor shaft '0 and the bevel gear 84.

Any number of planet gears may be used as may be necessary or convenient. the present arrangement, as particularly illustrated 'in Fig. 11, showing six double planet gears equally spaced about the cage I 24. This flxure also shows that the two portions of all but one of the planet gears are angularly oflset relative to each other to accommodate the teeth 01' the planet gears to the teeth of the ring gears with which they respectively meshsince the teeth of the ring gears will be correspondingly oii'set due tothe fact that the two ring gears do not have the same number of teeth. As is particularly shown in Fig. 11, numgear numbered I.

The movable ring gear 48 is operatively connected with the bevel gear 44 by suitable means, such as the spline connection I40, and the gear 84 meshes with all of the propeller blade turning gears, one of which is-indicated at I42 in Fig. 13

so that, whenever the gear .84 is rotated all of tion of the gears 84 and I in both directions and thereby limit the total pitch varying range of the propeller blades. These rings maybe set into an annular member I disposed between the end of the hub III and the adjacent end or the support ring :68 and secured to the-ring by suitable means, such as the screws I41. The rings may be held against rotation relative to the member I48 by suitable intermeshing teeth I48.

' One or more tapered pins, as indicated at I, having one end set into the hub I0 and the other end received in a wall in the member 68 maybe provided for properly locating the member 88 of each of the double planet gears meshes with during assembly and for resisting rotation of the member 88 relative to the hub member under the torque loads imposedby the pitch changing mechanism.

Suitable channels, as indicated at I52 andIII4 may be provided for the introduction oi. lubricant into the hub to lubricate the various elements of the pitch changing mechanism. These channels are disposed at diametrically opposite sides of the hub and are not connected but open into the hub at the point at which it is desired to maintain the level of lubricant in the hub. When it is desired to introduce lubricant into the hub the hub is positioned with one channel near the top and the other near the bottom. Both 01' the screw plugs I54 and I51 are then .removed and lubricant is poured into the top channel until it begins to run out oi. the bottom channel. The properamount of lubricant ,will then be in the hub and the plugs I" and IE1 will be replaced to prevent loss'oi lubricant through these channels. Whenever the pitch changing mechanism operates the lubricant is carried by the planet gears to the central gears and the various bearings. The i'ront end or the pitch changing mechanism may .be'enclosed by a suitable cover member or dome I58 which may be securedin place by an annular nut I" rigidly secured thereto. The dome and nut may be rotated by suitable tools inserted such as indicated at I62.

I It is to be noted that the'entire pitch changing mechanism including the motor, .the reduction gear unit, the support and the cover member can the propeller blades'are rotated simultaneously and coextensively to change the pitch setting of the propeller.

Suitable pitch limit stop means in the form of be pre-assembled to. constitute a complete unit bodily removable from the propeller hub and that this unit may be easily assembled with the propeller hub by locating the support ring 66 with the pins I and turning the nut 68 into operative position and locking it in place. This greatly facilitates propeller construction since the pitch changing unit can be bench assembled and tested and adjusted before assembly with the pr'opeller and is also ofgreat convenience in service since the pitch changing mechanism can be bodily removed from the propeller and replaced by a different complete unit and can then be over-.

Referring now particularly to Fig. 2, the motor 52 comprises, in general, a cylinder block I 84, the

sleeve member 56, a valve mechanism I 68, a pinrality of pistons I68 connected to a wobble plate I10 mounted on the motorshait by an antifriction bearing I12; an annular bearing support I14, and a fluid seal IfIlbetween the shaft 90 and the adjacent end of the member 5 and shaft bearing I11.

The cylinder block I is provided tral bore I18 within which the valve mechanism I is rotatably disposed, and with a plurality of apairoirings m and m having properlyspaced intapped holes with a cenof the cylinder so cylinder bores. one of which is indicated at I80, within each of which there is disposed a reciprocable tubular piston, as indicated at I68. At its end opposite the opened ends of the cylinder bores I80 the block I64 is provided with a cylin-- drical portion I82 which forms a seal fit with a larger diameter portion I84(Fig. 13) of the nut 50 screw threaded into the end of the shaft I6.

"and with a smaller cylindrical portion I86 which forms aseal with a smaller diameter portion I81 of the nut. The apertures 5I are between the larger and smaller portions I84 and I81'of the nut 50. A plurality of connecting. ports or apertures,'as indicated at I88, which open into an annular groove I90 surrounding and communicating with the central bore I18 are disposed between the cylindrical portions I82 and I86 of the block I64. Each of thecylinders I80 has at its closed end a Port I92 opening to the central bore 8.

plug member I93 having a closed outer end and two longitudinal channels I94-and I95 separated hy-a longitudinal partition I96 all rigidly secured. in a rotatable sleeve I98. The channel I94 opens to the end of the plug member adjacent the end of the tube 26 and connects with the end of v the tube .so that hydraulic fluid under pressure in the tube may flow into the channel I94. This channel also extends over the cylinder ports I92. The channel I95. extends over the cylinder ports I92- and the annular groove I90. Surrounding the I plug member I93 is the sleeve member I98 rigidly secured to the plug member by suitable means,

such as the rivets 200, and provided with a hollow reduced extension 202 within which is disposed a fluid pressure actuated piston 204 and a compression spring 206 opposing movement of the piston away from the position illustrated in Fig. 2. The

sleeve I98 is provided with an aperture. 208

through which the channel I94 may communicate with the cylinder ports I92 and with a second nel. A tubular member 2I2 surrounds the sleeve member I98 and has a driving connection "2I4' with the inner end of the hollow drive shaft 90 so The valve mechanism I66 comprises a rotatable the outer sleeve member in, the inner sleeve member I98 and the valve plug I98 with it so that the channels I 94 and I95 in the valve plu are brought successively into communication with position to supply the hydraulic fluid under presthe cylinders in which the pistons are moving outwardly while the channel I94 will be connected with the cylinders in which the pistons are moving inwardly. With this arrangement the hydraulic fluid under pressure maybe supplied either through the conduit 26' or through the space between this conduit andthe hollow shaft I6 and the alternative channel will be vented to drain depending upon the position of the governor valve. When the pressure fluid is supplied through the channel I94 and the chan-v nel I95 is connected with drain, the motor willrotate in one direction, and when the channel I94 is connected with drain and the channel I88 connected with the source of hydraulic fluid un- I der pressure, the motor will rotate in the oppo site direction. As is particularly shown in Figs.

5 and 8, each channel (I 94 or I95) is connected at all times with four of thenine cylinders shown, only one cylinder being blocked oil? at any one time, this being the cylinder which has its piston in top or bottom dead center position at the particular time and is thus in process of changing from intake to exhaust. Thus, each channel is open'to each respective cylinder for a time aperture 2| 0 of substantially the same extent as that of the channel I95 and overlying this chan-' that the tubular member and the valve mechanism will rotate with the shaft.

From an inspection of Fig. 5 it is apparent that the channel I94 will always be in communication with at least one of the-cylinder ports I92 so that pressure fluid may flow from the 'tube 26 to the closed end of the respective cylinder through this channel or alternatively exhaust fluid may flow to maintain the propeller speed substantially at the speed for which the governor is set. This fromthe cylinder to the tube and pump intake.

When the high pressure fluid is supplied through I the tube 26 the direction of rotation of the'plug I93 and sleeve I98 is such that the channel I94 is first connected with each cylinder while the respective piston is at or near the closed end t" fluid flowing into the cylinder will tend to force the piston outwardly.

"As each piston is forced outwardly it exerts a force on the wobble plate I10 tending to cause the wobble plate to assume a difl'erent angular position with respect to the axis of the shaft 88. Since the wobble plate is mounted on the shaft 90 by means of the antifriction bearing-I12 disposed at an angle to the axis of the shaft 88, the

, force exerted on the wobble plate by the piston interval corresponding to sli htly less than one hundred and eighty Edegrees of shaft rotation which corresponds toa piston movement from fully retracted position to the top'dead center position or vice versa. It is thus seen that the motor will operate with equal facility in either direction depending upon which channel supplies the fluid under pressure and which is vented to drain.

As long as the pitch changing mechanism is operating under governor control, it will limit the range of pitch angles to'the range necessary means that it the engine power is sufllciently reduced the propeller would go to its limiting low pitch position as determined by the low pitch stop comprising one of the rings I48 and I44. If the proper propeller is mounted on the engine, however, it will absorb the maximum power output of the engine at ahigh pitch setting at' which the blades still maintain the major portion of their propulsive emciency and the speed control will not, therefore, increase the pitch beyond this I68 will cause the shaft to-rotate about its axis in a manner well known to the art. As the shaft '90 rotates under the forceexerted on it by the piston I88 through the wobble plate I'll it carries maximum pitch angle during constant speed operatio If it is desired to change the propeller pitch beyond the constant-speed range to a feathered condition, the feathering pump 41 is put into operation. As this-pump builds up a pressure above the pressure produced by the engine driven pump 88, this higher pressure will act on the valve 48 to cut of! the governor from the lines 82 and "and connect the feathering pump through the'line 81 with th'e'valve channel I98.

Hydraulic fluid from the-pump 4I entering the channel I" will operate the motor- 52 and continue its operation until a maximum pitch or feathered condition of the propeller is reached the channel- I94 being connected with drain through the conduit 213. If desired, the operation of the pump may be discontinued at this point by some suitable device such as a pressure relief switch 2|! one form of which is particularly illustrated and described in United States Patent No. 2,333,978, issued November 9, 1943, by

Murray C. Beebe, Jr., for Electric feathering conasoassa draulic pressure in the closed end of the respective cylinder this piston will displace fluid in the space surrounding the wobble plate member and this displaced fluid will act to drive back into their cylinders those pistons whose cylinders are exhausting at the particular time. Thus, the pistons act in both directions on the wobble plate member and power is derived from the pistons on the exhaust side ofthe motor as well as from the pistons on the pressure intake side at any particular time. The fluid in the space surroundin the wobble plate member also serves to lubricate the wobble plate bearing I12 and the shaft main bearing-ill as well as the. connections between the various pistons and the wobble plate. Escape of this fluid through the shaft 90 is prevented by a partition 224 at the outer end of the shaft which location of the partition provides for the adequate position to the constant-speed control range of pitch angles, operation of the pump I may be resumed as by reactuating the pressure cut-out switch M5 or operating a different switch set for a higher pressure so that hydraulic fluid at an increased pressure will be supplied to the channel I95. As the propeller pitch changing mechanism cannot move further in the pitch increasing direction because. of the operation of the-positive high'pitch limit stop, pressure will build up inthe channel I95 and below the piston 204 through the port 2i6. When this pressure reaches a predetermined value the spring 206 will collapse permitting the piston 204 to move from the position shown in Figs. 2 and 9 to the position shown in Fig. 10, As the piston moves it carries with it a pin 2|! which moves in cam slots 22! in the sleeve I98 and in grooves 222 in rotation of the sleeve I98 and plug I93 relative to the outer sleeve 2l2 and bringing the valve channels from the position shown in Figs. 4 and 5 to the position shown in Figs. 7 and 8 thereby tinued when the blades reach the constant speed.

range of pitch angles or maybe continued until the propeller has reached the position determined by the positive low pitch limit stop and the pressure exerted by the pump 4| has increased suiilciently tn again trip the pressure operated switch and automatically discontinue operation of the pump and return control of the lubrication of the valve mechanism.

While a particularmechanical arrangement has been illustrated in the accompanying drawings and hereinabove described for thepurpose of disclosing the invention, it is to be understood that the invention is not limited to the particular construction so illustrated and described, but that such changes in the size, shape and arrangements of the various parts may be resorted to as come within the scope of the sub-joined claims.

Having now described the invention so that others skilled in the art may clearly understand the same, what it is desired to secureby Letters Patent is as follows:

1. In a hydro-controllable feathering-type propeller, a hub, a blade mounted in said hub for the outer sleeve member 2l2 thereby causing a v pitch changing mechanism to the governor 36 and pump 38.

The wobble plate member I10. and the pistons I68 are both provided with corresponding respective hook elements as well as with abutting surfaces at the ends of the pistons so that the pistons are secured to the wobble plate member against movementin both directions. This arrangement provides for an improved distribution of the forces directions by fllling the space surrounding the .wobble plate member with hydraulic fluid retained therein by the fluid seal I16 and also obviates the necessity of providing a fluid drain for the wobble plate chamber to take care of leakage past the pistons. With this arrangement, when one of the pistons is forced outwardly into th p e y rpitch chan in movements, a reversible multicylinder fluid motor in said hub, and a reduction gear train between said motor and said blade, said motor having a plurality of cylinders arranged in a circle, a piston reciprocable in each cylinder, a motor shaft, a wobble plate on said shaft connected with said pistons, a movable valve mechanism for directing the application of hy-v draulic fluid to the ends of successive pistons as said motor shaft is rotated, and pressure responsive means positioning said valve mechanism and operative to change the position of said valve mechanism and thereby reverse the direction of rotation of saidmotorshaft when subjected to a fluid pressure above a predetermined value.

2. The arrangement as set forth in claim 1 in which said valve mechanism is rotated by said motor shaft and said pressure responsive means comprises a piston and cam device between said valve mechanism and said shaft and a spring resilientlyresisting the effect of fluid pressure on said piston.

3. In combination with a hydro-controllable feathering-type propeller, having a hub, a pluralit of blades mounted in said hub for pitch changing movements, a multi-cylinder reversible fluid motor in said hub, a reduction gear train operatively connecting said motor with said blades to vary the pitch thereof a rotatable valve in said motor for distributing fluid under pressure to the cylinders thereof and providing for the exhaust of fluid from said cylinders, said valve having two fluid passages either of which may either conduct fluid under pressure or exhaust fluid,

speed responsive means for selecting the passage to its feathered condition, and pressure respon- =sive means associated with said valve for changing the relative position of said valve passages and said cylinders to reverse the direction of rotation 01 said motor to a pitch reducing direction .upon further application of pressure fluid to said motor through said one passage after-said 'ieathered condition has been attained.

4. A controllable-pitch-propeller having a reversible hydraulicaliyoperated pitch changing motor, and two hydraulic lines for supplying hydraulicv fluid. under pressure to one side or the other of said motor to operate it in one direction or the other, a overnor actuated valve for selecting the line through which pressure fluid is sup.- plied to said motor, a first source 01' fluid under pressure connected with said governor actuated aseasoa valve, a second source of fluid under pressure, a valve located between said propeller, said gov-' ernor and said second source of fluid under pres- I by pressure developed by said second source of fluid under pressure in excess of a predetermined value to change the connection between said one hydraulic line and said'motor to reverse the direction in which said motor is operated by-presthrough said one line.

FRANK W. CALDWELL.

EDWIN E. VINCENT.

sure fluid supplied 

